


In Different Worlds (SenHaku Week 2021)

by Lacie_Noirflecet



Category: Dr. STONE (Anime), Dr. STONE (Manga)
Genre: Alternate Universe, Alternate Universe - College/University, Alternate Universe - Modern Setting, Alternate Universe - Modern with Magic, Alternate Universe - Regency, Alternate Universe - Roommates/Housemates, And Cupid! Gen at the end, Angel! Ruri, Cross-Posted on FanFiction.Net, Cupid AU, Cupid! Kohaku, Domestic Fluff, Earl Kokuyo, F/M, First Kiss, Forgive Me, Gen has telepathy, Ginro (mentioned) - Freeform, Human/Vampire Relationship, I think Ruri would take Literature, I tried but i'm being called to sleep, I'm so sleepy for the last theme, Ishigami Byakuya (mentioned) - Freeform, Jealousy, Joel (mentioned) - Freeform, Kaseki (mentioned), Kinro (mentioned), Kohaku has so much potential when it comes to courses and programs I swear, Kohaku is taking Radiologic Technology, Lillian Weinberg (mentioned), Luna Wright (mentioned), Mages, Marquess Senku, Marriage, Mission Fic, Nanami Ryusui (mentioned), Other Additional Tags to Be Added, Regency, Rei is Senku's older sister, Rei would take robotics!, Roommates, SenHaku Week (Dr. STONE), Senku being rarely sweet, Senku is taking Aerospace Engineering, Suika (mentioned) - Freeform, Valentine's Day, Vampire Senku, Witches, Wizards, domestic AU, kinda turned Magma and Mantle as sons of earl too, kisses under the starry sky, that means Lady Kohaku and Lady Ruri, where they have powers!, with a bit of Vampire AU
Language: English
Status: Completed
Published: 2021-02-09
Updated: 2021-02-15
Packaged: 2021-03-15 05:34:43
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 7
Words: 7,801
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29308887
Author URL: https://archiveofourown.org/users/Lacie_Noirflecet/pseuds/Lacie_Noirflecet
Summary: There are plenty of worlds out there.There were plenty of scenarios in which Senku and Kohaku could meet. Maybe, they would miss each other once or twice.But, in the end, it was always them together.(Alternatively, SenHaku AUs for SenHaku Week 2021!)(Completed as of February 15)
Relationships: Ishigami Senkuu/Kohaku
Comments: 8
Kudos: 35





	1. Mission AU: During The Ceremony

**Summary for the Chapter:**

> A SenHaku AU where Kohaku has to get married to someone for a mission and Senku watches.

There were many words to describe Kohaku.

They'd look at her as a strong gorilla with excellent eyesight. Others would think of her as a lioness that will pounce on you if needed. But today, the perfect way to describe her was a fairy queen mingling with mortals.

Kohaku walked down the aisle with a bouquet of orchids on hand. She decided to wear a white dress rather than the traditional puffy ones. The blonde complained that a gown would hinder her movements for this occasion.

Senku attempted to hide a chuckle as she saw her coming. Of course, she needed to be able to move with ease in case shit hits the fan. They weren't there to watch Kohaku get hitched. They were there to proceed with the mission.

The plan was for their most lethal member to catch the attention of the prime minister's son. The two gets married then they'll have an insider without much effort. Phase one was already a success.

They only need to make sure phase two will end smoothly.

"If anyone objects to this marriage, speak now or forever hold your peace!" The priest exclaimed.

"Senku-chan, wouldn't you want to sweep Kohaku-chan off her feet?" Gen telepathically said, almost in a sing-song voice.

Everyone who was part of the mission heard it. Kohaku sent a glare to the best man, daring him to say another word about her predicament. Ukyo warily glanced at Senku as he waited for a denial. The others didn't dare to react to it as they tried to focus on the current situation.

"Letting emotions influence our mission will ten billion percent cause us nothing but trouble," Senku replied through the telepathic link.

Everyone would have openly sighed if they didn't have to keep their act. It was just so natural. There wasn't a surprise there even one bit.

"Heh, but I wouldn't mind marrying the lioness in his stead."

The scientist could have sworn Kohaku stuttered with her vows.

**Notes for the Chapter:**

> Hi! It's been a while since I wrote a fanfic, and I tried to do it again after a long time.
> 
> For this one, I thought it would be nice if Senku and Kohaku had to get married for a mission... until I changed my mind. I thought why not put them in the same agency and Kohaku is the one getting fake married? So, I ended up with this.  
> I really wanted to showcase everyone's powers in this AU, but Gen is the only one who gets to because of telepathy. So, if you're wondering, Ukyo is an empath while Senku has wind control. Kohaku can take any characteristic of any animal.
> 
> I hope you all enjoyed this!


	2. Modern AU: Those Rare Times

**Summary for the Chapter:**

> Day 2: Jealousy
> 
> A Modern AU in which Kohaku rarely wanted something with Senku.

Kohaku accepts Senku for who he is.

She knows his personality well, and got to know him more the longer they were together. The blonde wasn't a mentalist like Gen. However, the time spent together was enough to figure out the budding scientist's tiny tell of emotions.

And, Kohaku has long accepted his way of expressing love. It was always a small but thoughtful gesture. She wouldn't deny that it would make her heart beat faster than the times during her competitions.

Senku's gestures were never as flashy as the couples she'd see in the ever-bustling Stone Cafe. It wasn't grand and cheesy. She couldn't have ever been happier since they both never liked a fuss.

But, there are times she wanted more.

Kohaku knew it was selfish. She shouldn't ask for more because she really has this great relationship with Senku. They are going strong and, even with the long-distance relationship they sometimes have, everything was perfect.

All it takes was to be observant of her surroundings to make her feel uncomfortable. The couples in the cafe would be snuggling publicly to warm themselves. Joyful laughs and unrestrained giggles blended with the cafe's music. She could even hear the sweet weekend plans from the neighbouring table.

Kohaku sighed as she laid her head on her table. She couldn't even focus on her homework because of the couple a table beside hers. They weren't doing anything wrong. They were only talking over coffee about the promises of amusement parks and aquariums.

With Valentine's Day coming in a few days, she couldn't help but be jealous over the couple's plans.

The details the two has laid out were enough to make her imagine the date. The couple was going to pack a bento for lunch at the amusement park. They were going to share an ice cream for dessert. There was a laser tag game after the rides. Before the night ends, they were planning to try the fish spa at the aquarium. The dinner even sounded so good too!

Kohaku knows she doesn't act like a girl sometimes. But, still! She was one deep inside!

And, she wanted those dates too from time to time!

At this point, she wasn't sure anymore if she was jealous or frustrated. Maybe, it was both. It also could be something else.

The bell rang, signalling a new customer of the cafe. Kohaku snapped out of her reverie and became more aware of her surroundings. She stared at the table filled with pens and notebooks, and a mocha-filled cup.

She never really touched the coffee Senku made, huh?

"Rei, you're late," Senku called out from the counter.

The woman laughed. "Sorry, Senku. I had to drop by the university."

"Did the old man forget something again?"

"Just a few papers as usual. Give me five minutes to switch with you." Rei stated as she disappeared into the staff room.

Kohaku watched the two siblings interact. It wasn't uncommon for the scientist to manage the cafe for Rei every Wednesdays. Today was just for a long time given the woman was out on a date. And like every other Wednesday, she and Senku would soon leave the cafe and walk home.

The blonde realized she was becoming more lost in her thoughts today. The moment she returned to reality, Senku was already looking at her. Flustered, she immediately busied herself with her forgotten homework. She could feel him approaching her table.

As she tried to focus on the complicated question, the seat in front of her became occupied. "Lio-"

"Not a lioness," was Kohaku's automatic reply. "What?"

"Do you want me to reheat your coffee?" Senku asked.

Finally, the blonde looked up from her homework. "Yes, please."

Senku got up and took her cup without a word. She smiled, following him with her eyes.

From her seat, Kohaku noticed the next table couple starting to get up and leave. She watched them step out of the cafe. The couple shared a kiss before the girl's taxi drove away.

The blonde sighed. _It must be nice._

* * *

"I'm surprised you were able to stand the couple back in the cafe," Senku stated as he walked her home.

Kohaku shrugged and took a sip from her coffee. She was able to drink a considerable amount compared to when she was in the cafe.

The junior scientist wasn't dense. He knew something was up ever since the couple stayed in the cafe. They weren't loud during their stay, but the lovey-dovey atmosphere was apparent as hell. Everyone would be able to notice it a mile away. It wasn't an understatement.

He knew the cause of the lioness' attitude. The next thing he needed to do was to find out-.

And before Senku knew it, he felt a warm hand grasp his. He'd flinch in a normal situation. He could have shaken it off like when somebody tries to hold his hand. But, this was Kohaku who tightened her grip on him.

"What happened, Kohaku?" He decided to ask. He might be a genius scientist, but he wasn't a mind reader.

"It's nothing." The girl averted the doubtful red eyes. She tried to find the right words, but she failed. Honestly, she felt a little bit embarrassed to admit what she wanted.

When he knew she wasn't going to answer yet, the scientist ended up sighing and sticking a finger in his ear. He knew she was stubborn. It was already proven when she and her father had some problems two years ago.

"I don't think it's nothing, lioness." Senku switched back to her nickname. And as if the cup of mocha was liquid courage, Kohaku suddenly decided to down the whole drink. "Kohaku?!"

Ignoring the surprise in his voice, she turned to him and took a deep breath. "I want to go on a date with you on the weekend!"

"Huh?"

"I want to go to the amusement park or the aquarium! If you don't want to, we can try the skating rink or the laser tag arena! I want those kinds of dates too like what those two were planning! I want to be lovey-dovey too even sometimes!"

Silence dominated the two as fast as Kohaku expressed her desires. While the girl was trying to catch her breath, Senku put two and two together. His brain seemed to process things slower for him today. It was uncharacteristic of him, but it does happen.

Finding himself speechless in this situation, the scientist pulled the lioness closer to him for a hug. He'd be able to say something usually. It was rare, but there were times he'd let the illogical part of himself running around.

It took some time, but she did return the hug. When she did, he couldn't help but hold on to her tighter. He smiled. "Were you feeling jealous of those two?"

"Hah! As if!" She denied and her hug on him got tighter.

After a few more minutes, they parted slowly as if not wanting to let go of each other. They would have hugged more if they weren't on a public road. However, their hands did not separate.

They continued holding hands for the rest of the walk with the unease gone from the two of them.

**Notes for the Chapter:**

> Lacie, here!
> 
> Today's chapter was supposed to be a Cafe AU, but it took its own path before I knew it. I really had lots of fun writing this chapter. It took me some time to write it, but I hope you guys enjoyed it.
> 
> I'll do my best again for the next chapter!


	3. College AU: Behind Closed Doors (And One That Wasn’t)

**Summary for the Chapter:**

> College AU where Kohaku and Senku deal with being roommates.
> 
> Day 3: Roommates

Stone University was the most prestigious school in the country.

The school with their top-notch classes gave birth to genius professionals. That’s why a lot of famous people from various fields came here. Not only did they have tough entrance exams, but the freshmen year was also hellish. But after the gruesome first year, the students could customize a schedule fit for them. Some would say the school had a complicated system.

However, the dorm was more unpredictable.

The students were required to live in the residence hall after the gruesome first year. The rooms were spacious enough to do anything they want. There was a kitchen, a bathroom, beds, closets, and a vacant space. It seemed perfect, but the administration has a way of matching roommates randomly.

It was random enough that two people were having a hard time figuring out the current arrangement.

"Lioness, do you know why they paired us up?" Senku asked as he set down the last of his boxes.

"I'm not a lioness!" Kohaku huffed. "I don't know. I was supposed to be with Ruri." 

"I never heard siblings in different years being roommates."

"There was one before!"

"They were twins." The scientist stuck a finger in his ear and observed the red-faced blonde.

In the end, she turned back to unpacking the rest of her luggage.

They were definitely off to a good start.

* * *

The two roommates were already familiar with each other before they lived in the same room.

They came across each other frequently despite being in different programs and clubs. Ruri and Rei were to thank for that. It was the same for their friends. Still, they weren't able to really interact a lot back then. It was only until they became roommates that they got to know each other. Not just their quirks, but also their different studying habits.

Senku glanced from his tablet when he heard a loud thump. Kohaku was troubled again. He knew from the six months of being together that she'd lightly hit her head on the table when she was frustrated with her studies. Tonight was one of those times. He couldn't blame her. Cramming for the finals have always been ten billion percent a pain.

However, he was wondering why did the girl choose Radiologic Technology as her course out of everything. With how active she was in sports clubs, he almost thought she was in Sports Science. She was very involved in the school. A small smile found its way to his lips. He really admired her ability to juggle her academics and extracurriculars.

"Senku," Kohaku called out. She turned her head to face him. "What's wrong?"

_ A perceptive lioness, as usual.  _ The boy adjusted from his seat before asking. "Why did you take your course?"

The girl sat up straight and Senku couldn't help but notice how her blue eyes were full of emotions. "We had to rush Ruri to the hospital before. The radiologist there discovered my sister had pneumonia. If we found out late, she would have died."

"Ruri has been doing well already."

"And, I'm happy about that." Kohaku smiled. "I wanted to be like that radiologist. I wanted to save people like her."

The two settled into silence once she was done speaking. There were no more words to be said. It wasn't awkward. It was... comfortable.

The night continued with a peaceful silence around them.

* * *

By the time they hit their junior year, their schedules were different from each other.

Senku's timetable was filled with classes in the morning. By the time it was afternoon, he was either in his club or in their room. Her roommate was mostly spending his time crafting a spacecraft. Whether it was his own choice or because his program demanded it, he was occupied.

Kohaku was busier. She alternated her schedule between classes and clubs. The girl claimed that staying still and listening to all the lectures were going to drive her crazy. It's why she opted to squeeze her clubs in the middle to get some fresh air. She even had to practice for competitions from time to time.

The two developed a routine throughout the time they spent. They always had dinner together despite their busy schedules. If they couldn't, they'd grab a snack at least.

After coming from a long tiring kendo practice, the lioness desperately needed a shower. She opened the door to their room and was greeted by the cold air. Her grip on the box full of mille-feuille tightened as she shivered. She couldn't let the box drop to the floor. It was their snack before Senku leaves for dinner with Byakuya, Lillian, and Rei. 

"Senku, it's too co-" Kohaku stopped upon seeing the Aerospace Engineering student asleep on his desk. The afternoon sun was the only thing illuminating their room, and it created a paradisical image to their space. She would have continued to be in trance at the scene in front of her if it wasn't for the soft rustling sound of the papers on the floor.

Placing the box of sweets on the kitchen counter, she tiptoed to his bed and took the neatly folded blanket. The girl unfolded it with a flourish sending some papers flying. She'd pick that up later. For now, she should make sure her roommate won't be freezing by the time she finishes her bath.

She was fast to clean up the room. She had to. There are two more hours until his family arrives.

As she was about to get into the bath, courage struck her. The blonde glanced back to the sleeping figure by the window. The silly thought in her head pushed her forward to do something uncharacteristic. With steps as light as a fairy, Kohaku hurried to his side and planted a gentle kiss. She practically ran to the bathroom after that.

And if Senku felt her lips on his exposed forehead, he didn't dare to open his eyes and speak of it.

* * *

When their senior year started, the roommates already gained an almost perfect understanding of each other.

That included the awful luck one student has.

Days before his thesis defence, Senku just had to be down with a fever. It wasn't the kind that will immediately go away with a good night's sleep. He struck gold with the one that made him incapacitated to move. On the bright side, he didn't have to prepare for the defence.

"I'm sorry, lioness." The engineering student weakly apologized.

From the small kitchen, Kohaku resisted the urge to snap at the nickname. It wouldn't do them good if she did. He needed rest more after 'accidentally' overworking himself. He should conserve his energy more.

The blonde decided to focus on cooking the ramen. No matter how sick he was, he always had the appetite for it. It's one of the reasons why she decided to learn to cook it from him. The taste wasn't like the one they bought in the ramen house an hour away from the university, but it was close enough.

Taking care of Senku was definitely a piece of cake for her. She already did this many times not only to him but also to her sister. When the food was cooked, she made sure he was propped up and the ramen wasn't steaming hot for him to eat. They ate in silence with the lioness having to spoonfeed him.

Once they were done, Kohaku got busy cleaning up. The earlier she could get the boy to rest, the better. She still had to smooth things out for her practicals. He might be almost done with his requirements, but she still had two to fix.

"Kohaku?" Senku called her name as she tucked him in.

The lioness swore her heart skipped a beat. "Yes?"

"Thank you." And with that, the boy placed a tiny kiss on her forehead and he sleepily chuckled. "I remember that too."

No more than a minute did he fall asleep, leaving her shocked from his actions. She held her face as a blush slowly crept to her face. Memories of the stunt she did a year ago rushed to the surface, and she felt more embarrassed than ever.

And if Senku felt her kiss from before, he did dare to speak of it while in a state of a delirious fever.

* * *

Stone University always had a graduation party for graduates.

It was an event where alumnae and outsiders were welcomed to wander around the school's residence halls. Some freshmen would take this opportunity to have a sneak peek of their stay. For the seniors, it was their last opportunity to have a feel of their second home.

Kohaku and Senku stood in the middle of their room, taking one last look at everything before they start packing tomorrow. They weren't the type to feel melancholic. It wasn't like they couldn't help it. There were so many memories of their stay here.

Honestly, they still didn't know why the school administration decided to put them in the same room.

"Senku!"

"Kohaku."

The graduates turned their attention to the source of the voice. They found Rei and Ruri smiling at them from the doorway.

"Ruri? Rei?" Kohaku tilted her head to the side.

"We knew you'd be here!" Rei answered as she bounded to Senku with a bouquet of hydrangeas. "After all, we picked out this room for the two of you!"

"We hope you had a lot of good memories here. Congratulations on graduating." Ruri casually said.

For a while, the two roommates weren't sure if they heard Rei correctly. They turned their heads toward each other and looked back at their older sisters. If Ruri and Rei picked out this room for them, then...

"Rei, did you just say..." Senku trailed off.

"We picked out this room for the two of you!" His older sister repeated, but this time with a more cheerful voice.

"But, how?" The younger blonde exclaimed.

"As part of the student council, we had privileges." Ruri glanced at Rei. "Since Rei and I were friends, we thought the two of you might get along well too."

Silence.

The room was so silent you could hear a pin drop. It was a needed silence though. Whatever theory Senku and Kohaku had before, it was proven wrong in an instant by their sisters. 

"All this time, we believed ten billion percent that the administration was behind it." Senku chuckled. "I didn't consider the student council. That explains why the school got crazier when Ryusui became president."

"Ruri, why didn't you tell me?" Kohaku asked her sister.

Ruri made an apologetic face. "Sorry. It was supposed to be a surprise."

"Anyway, you're missing such a great party downstairs!" Rei pulled on Senku's hands. "Let's go."

"Rei, can you give us a minute?" Ruri and Rei glanced at each other until the older blonde smiled knowingly. 

Kohaku smiled when their older sisters left. It would probably be a long time before she and Senku would be able to meet again. They already have promising futures ahead of them. They would be busy with their lives.

Before any of them could say a word, the two closed the distance and tightly hugged each other. Three years. It took them three years to have this kind of hug.

It wasn't the end, but they might as well start with one hug.

**Notes for the Chapter:**

> Hi! Lacie here!
> 
> The prompt for today really gave me a difficult time, but I survived. While writing this story, I had to make a couple of researches. Kohaku has so much potential that looking for a course/program for her was really challenging! At first, I was thinking of Nutrition and Dietetics or a social sciences program for her. I completely changed my mind when I discovered that you can detect pneumonia through radiology. In the end, I decided a course close to radiology.
> 
> I hope you were able to enjoy today's chapter! Have a great day! ^o^


	4. Regency AU: The Right One

**Summary for the Chapter:**

> A Regency Era AU where Kohaku is the earl's daughter and Senku is a marquess.
> 
> For SenHaku Week 2021 Day 4: First Kiss

"You mustn't simply give your first kiss to anyone, Kohaku. It should be with the right person." Was what her dearly departed mother once told her.

The girl took those words to her heart and carried it everywhere with her. It was like a promise. She reserved her kiss for someone worthy. She never let a boy kiss her, and, even though it was improper, she would slap their faces.

Of course, her father looked horrified when she did that to her suitors.

Kohaku won't get married at this rate. My daughter will grow old by herself. A lady like her will never find a lord or duke because of her personality. Earl Bluebelton's youngest is too feral. The family's only hope is the eldest.

There were so many words she heard from her father and other people. She can deal with the judgement of others. People would never know the truth behind their words. They couldn't see what was beyond. But, her father's?

It was too painful.

Was there something wrong with keeping her word to her mother? Is it a mistake to protect herself from those opportunistic men? Did her father not know her mother's requests to her and Ruri? Did he not want them safe and sound?

Kohaku clenched her fists tightly. _It's so unfair._

"Milady, is there something wrong?" Kujaku asked, stopping to brush the girl's hair.

Kohaku looked at the woman from the mirror and smiled politely. "Everything is fine."

"Your father wants you to meet someone later. He said he thinks he finally found you and your sister good suitors." The short-haired woman stated as she went back to her previous task. "However, if I may speak my mind, they're not enough."

"I guess that's something we shall see."

* * *

"Kohaku, you'd behave well for today," Kokuyo ordered his youngest as headed to the parlour.

"Father, I don't think Kohaku would do something unacceptable." Ruri softly stated, but the words fell on deaf ears.

Kohaku could only look down in silence.

When they arrived at the parlour, their father smiled at the waiting guests. "Welcome! Thank you for joining us."

Both the siblings curtsied in front of the Earl's sons. Kohaku looked up to take a quick once-over at their guests. Her eyes widened as blue met blue. She recognized the lord in front of her. She knew him, and she wasn't happy with this meeting.

"Lord Magma, Lord Mantle, shall we?" Kohaku could hear her father speak, but her gaze was on the large lord she knew too well.

As if she was going to let the two of them become part of their family.

* * *

Kohaku couldn't believe it.

Kokuyo immediately sent her to her room when he noticed her defiant attitude towards the Earl's sons. She had to chase them away. And knowing Lord Magma's personality, she knew she had to make an extra effort. Her image be damned. She was going to protect her older sister!

Feeling frustrated, Kohaku grabbed her cloak, took her horse and snuck out of the manor.

She needed a breather, and it was impossible to have that inside the house. She had to go _somewhere_. She had an idea where, but anywhere but this place was fine. The lady urged her horse to move faster. She trusted the stallion would lead her to that place.

* * *

Kohaku had gone to the marquess' house several times before.

She and Senku Ishigami first met before her big introduction to society. It was during one of the times she sneaked out of the manor. She found him by accident trying to make something fly up into the air. Upon seeing his crafts, she became fascinated with it and frequently came back to watch. He found her soon after.

When they saw each other at her debutante, Kokuyo wondered why they seemed close. He was also the reason why they had to dance together for one song. It wasn't bad. Kohaku would describe it as a pleasant experience. However, it was only good because that day was full of men trying to pursue her. A duke's son even tried to kiss her, but she dared to punch him in front of many.

The incident became part of the morning news.

"Lioness, I thought you weren't leaving the house today," Senku said as she approached him.

"I'm not a lioness!" Kohaku sighed. "Father found a suitor for Ruri. I made a scene and got kicked out of the dinner table."

"I can't see anything new there. Who was it?"

"Lord Magma." She intentionally left the part that her father thought she could be a match for Lord Mantle.

"Hmmm... Your father seems to be getting desperate in finding suitors."

"As you said, _marquess_ , there was nothing new," Kohaku repeated his words. She looked over to what he was holding in his hands. "What's that?"

Senku chuckled and looked at a part of his latest invention. He only needed the piece he was holding in his hands. "Rather than explaining, why don't you see it first?"

She blinked in confusion, but still nodded as she trusted him. The boy grabbed the lamp he was holding, and they crossed the fields together. It was a long, silent walk as the marquess' land was expansive. Despite having light to illuminate their way, there were times that he would trip. It's why Kohaku would always hold him for support and not the other way around.

Finally, they arrived in front of an enormous tree. It wasn't like the others in the land. It was one of a kind. With the lady's impressive eyesight, she could see that something shined on the trees whenever the moonlight hit it. What was it? She wasn't sure. It was probably another thing the boy crafted.

Senku removed himself from her side and walked towards the tree. She could see him tinkering with a box. Her heart was beating with excitement. She was looking forward to whatever that was. His skill to diligently work on his inventions made her have respect for him. Maybe that's what made her hang around him too.

Not only was she amazed by his scientific endeavours, but his personality was unlike other men.

"I'm going to turn this on now," Senku stated.

As he pushed a button, the tree lit up, and Kohaku couldn't take her eyes off of it. There were so many specks of light decorating the tree. Nature even decided to make it better by drizzling snow from the heavens. She could have sworn she saw a faint rainbow of colours. The scenery was absolutely breathtaking.

It was probably even the most beautiful thing she has ever seen.

From beside her, Senku smiled. She didn't even notice him walking to her. He noted how her shoulders relaxed as if her previous worries faded. A smile graced her lips, and her eyes lit up far brighter than the lightbulbs he made. Kohaku was supposed to say something to him. However, as soon as she looked at him, his gaze caused her to forget her words.

Everything seemed to vanish as they stood there caught in a spell by the brilliant glowing lights.

They took in each other's features memorizing each detail to heart. In the end, their gazes landed on each other's lips. They closed their eyes. The distance between them slowly shrunk with Senku cupping her face. Kohaku held onto his coat, and like that they kissed each other softly.

Years later, Kohaku would be able to say she gave her first kiss to the right person.

**Notes for the Chapter:**

> Lacie here~!
> 
> Day 4 of SenHaku Week 2021 was First Kiss. Since the AU was set in the regency era, it wasn't the time where lightbulbs were invented yet. That's why we have Senku who has now invented it! It's Kohaku's first time seeing one so, that was a challenge to describe somehow.
> 
> I hope you were able to enjoy it! See you in the next chapter~. ^o^


	5. Magic AU: To Scientists And Mages

**Summary for the Chapter:**

> A Modern Magic AU where witch!Kohaku helps out scientist Senku make a device for Ruri's sake.
> 
> For SenHaku Week 2021 Day 5: First Time

Scientists and mages were very different from each other.

One side was focused on seeking answers and developing the world. They used science and became the rising figures of the modern world. Meanwhile, the other was pushed to the shadows and, yet, they still performed miracles with their magic.

They clashed at times, but the peaceful moments between the two dominated the bad ones.

"What's that?" Senku asked as he saw his apprentice fiddling with a phone.

"I'm trying to see if I can make this phone store some really bad magic." Chrome huffed. "I think you need sealing magic for this, but I can't be so sure."

It was only about a month ago that Chrome decided to learn under him. Senku didn't believe that mages existed until the brown-haired boy showed his powers. The scientist became convinced right away after he couldn't find any concrete evidence of the little act.

Chrome wanted to find a way to merge science and magic. From that wish, Senku decided to build a group focused on this kind of study. He was also curious anyway. So, why not?

They started gathering data from both sides, but it was challenging. Chrome and Joel were the only ones who were studying both magic and science. While both boys were brilliant, they didn't possess a deeper understanding of sorcery. The lack of knowledgeable mages in the team was becoming a problem.

Acknowledging the issue at hand, Senku decided to add more people to the team. "Do you know any powerful mages that can help us?"

"I know a really bad gorilla."

* * *

Senku was slightly expecting a talking gorilla when Chrome said he knew a gorilla.

However, there was no talking gorilla. There was only an American looking girl frowning at him. Rather than a monkey, the scientist thought she looked more like a lioness because of her hair. Was it also magic? It seemed pretty odd.

"This is Kohaku. She's the gorilla I was telling you about!" Chrome introduced her.

"I'm not a gorilla!" She exclaimed as she hit him on the head several times.

"Ehhhh? She seems more like a lioness." Senku commented, sticking a pinky finger in his ear.

"I'm not a lioness either!" The girl protested once more. She approached and scrutinized him. "Chrome, he doesn't have magic."

"I'm only a normal human being." The scientist held out his hand. "It's nice to meet you, Kohaku. Your help will ten billion percent get us far in our research."

She blushed when she heard him utter her name. But instead of shaking his hand, she turned to Chrome. "The coven won't be happy with this. You'll lose your chance to be with Ruri at this rate!"

"But, what if we need to combine magic and science to save her? I had to take the risk!" The scientist's apprentice argued.

It was Senku's first time to hear the other boy's real intention. He knew he was desperate to merge the two subjects. He wasn't the type to pry. Chrome had his secrets to keep, but the lioness opened the topic that he couldn't help but feel curious.

"Fine." She said before she walked out.

That was the first time Senku met Kohaku.

* * *

After that incident, Chrome told him everything.

It turns out Kohaku's family was one of a kind. They were a long line of witches and warlocks whose blue eyes could clearly see the magic around them. That was the reason the girl knew he didn't possess any magic.

As the family had plenty of magic, they headed the coven Chrome belonged. The current head was Kohaku's sister, Ruri. She was powerful, but her body couldn't take the amount of magic she kept inside. It was killing her. The only solution they could find before was to transfer her magic into crystals. However, it wasn't enough.

Chrome's proposal was a peculiar idea - contain Ruri's magic using a science invention reinforced with fortifying spells.

The boy almost got kicked out of the coven because of the suggestion. Because of it, he started studying science and ancient spells in secret. He understood sorcery since he grew up in a magical family. It was then he knew he needed a science teacher.

Senku was up to date after that conversation.

The scientist knew they would need help from the coven. The issue was that they might not be willing to help. Another option was having an influential mage in the coven added to the team.

And the only one who matched the description was Kohaku.

"Senku." Chrome knocked on his door. He sidestepped to reveal the blonde.

"I'm sorry." She bowed her head. "Please let me help you save my sister."

With a smirk, Senku once again held out his hand. "Welcome to the team."

Kohaku shook hands with him this time.

* * *

It's been months since their team increased in size.

Their team consisted of elite scientists and two scientist-mages before Kohaku joined. One little witch and four warlocks from the coven became members soon after. It didn't seem much, but the researched quickly progressed because of it.

Chrome offered a fresh perspective on tackling both science and magic. Senku and Xeno worked mostly on science. Joel and Kaseki focused on crafting the container. Ginro, Kinro, and Suika provided information on different kinds of magic. Kohaku and Ukyo's abilities allowed them to check the results. And with their collective efforts, they were able to create a prototype.

They only had to go out and test it.

"Kohaku, is this place really okay? Shouldn't we go somewhere else?" Chrome asked as he looked around the forest.

"It's perfect. No one will come here, and it's a good magic hotspot." Kohaku answered. "Besides, Ukyo would know if someone is heading our way."

Ukyo nodded. "The magic here is compatible with Kohaku. I also cast an invisibility spell just in case."

"Let's start the experiment then," Senku announced, holding out the circular device to the witch. "Lioness."

"I'm not a lioness." She took the device and walked away from them. "You might all want to step back."

And, they did.

Kohaku recited a few spells, and, before anyone knew it, the wind picked up around her. The leaves were violently plucked from their branches due to the intensity. The water from the pond floated and created violent torrents around them. They even felt the tiny vibrations of the ground from where they were standing.

A disaster was possible if the girl wanted it.

However, the image immediately changed. A simple wave had calmed mother nature. A flick of Kohaku's wrist and the wind gently blew in the direction of the device. Even the water softly flowed like small streams.

It was during that time that the serene expression on Kohaku reflected her magic.

"Miss Kohaku is certainly elegant," Xeno commented from beside Senku.

Senku wasn't the type to let his feelings surface. He wasn't also one to express his emotions. Yet, he couldn't help but nod in agreement.

He never believed in fairies, but he thought he saw one today.

* * *

"Good work for today."

"We'll be taking our leave now."

"Good night, Senku, Kohaku."

"Take care on your way home!"

With that, Senku and Kohaku were left alone in the laboratory.

After finishing the device, they decided to have a drink in the laboratory. Everyone drank as a celebration of literally being a step closer to their goal. There were no more tests to run anymore, and the days of making a prototype were long gone. They didn't have to work late starting the day after.

The only thing left for tomorrow was to have Ruri use the device.

The scientist and the mage continued to sit in silence. Both were staring at their full wine glasses. They weren't sure how many bottles of wine entered their system. However, the amount still didn't feel enough.

It felt like something was missing.

"I wasn't expecting to work with a scientist," Kohaku said as she tried to dispel the silence.

Senku chuckled. "And, I wasn't expecting to work with a lioness."

She didn't dare to comment on her nickname this time. Instead, she raised her glass with a smile. "To scientists and mages."

"To scientists and mages." Both of them made a toast and sipped at the warming beverage.

Now that they're hours away from accomplishing their goal, the two opened themselves. There was so much tension between them in the past months. They always pushed it down that they weren't sure when it started. In a heartbeat, they closed the distance between them. Logic and thoughts flew right out of the window, and they let their bodies do the talking.

Needless to say, no one heard anything from Senku and Kohaku until the next day.

**Notes for the Chapter:**

> Lacie here~!
> 
> The theme for SenHaku Week 2021 Day 5 was First Time. Coming up with an idea was difficult at first. But, when I started writing it, I had some fun! I just had to include Xeno and Ukyo for this chapter. I thought that Xeno would help Senku with these things.  
> I have this headcanon for Ukyo that despite not being able to see magic like Kohaku and Ruri, he would be able to hear it.
> 
> Anyway, I hope you all enjoyed it! Have a great day and see you tomorrow for the next chapter!


	6. Cupid AU: Shooting A Love Arrow

**Summary for the Chapter:**

> An AU where Kohaku is a cupid who is trying to shoot Senku with a love arrow.
> 
> For Day 6: Valentine's Day of SenHaku Week 2021!

February has always been the busiest month for cramming cupids.

As angels of love, cupids had a long list of people to match. They had to make sure those people got together on or before Valentine's Day. If they were unsuccessful, another cupid would have to try again next year on that couple.

Kohaku was an excellent cupid. She was able to complete her list before February came every year. Her expert marksmanship allowed her to move on from one case to another without any problems. It's the reason why she could take breaks before the next list came.

She wouldn't have a break for this year though.

Two months ago, she started working on her last case. The final 'couple' on her list was a scientist named Senku Ishigami and a medical student called Luna Wright. The cupid thought they would look great together. So, she cast a love spell to her arrow and shot it straight to Luna's heart during a science expo. The American instantly became head over heels for him.

However, Kohaku's only mistake was to disguise as a mortal.

Senku spotted her before she could shoot another arrow. She kept her presence low, but he still felt her there. She couldn't risk being discovered in public back then. So, the angel pretended that she attended the expo to see him as a fan. It came as a surprise that the scientist didn't believe her and knew she was a cupid.

She later discovered his stepmother was an angel.

Despite being discovered, Kohaku didn't stop trying to accomplish her duties. She tried following him inconspicuously. Senku kept on finding her and made it impossible to shoot him. At some point, she was so frustrated that she pursued him openly.

It didn't make any difference. _At. All._

Tomorrow was Valentine's day already. It was her last attempt. She had to do it before the 15th.

However, Senku had a different plan. The scientist asked her if she was busy. And, she was.

_She was going to be busy trying to shoot a love arrow to his heart after so many failed attempts._

As if she would say that. So, the cupid denied her true intentions, and he decided to drop a horrible suggestion. He invited her to a date. Her! A _cupid_! The same one who was _trying_ to get him together with the pretty American since _day one_!

"What am I going to do, Ruri?" Kohaku asked her older sister after sharing her current problem.

"That is a problem," Ruri stated while she patted the other girl's hair. "Do you like him?"

"Ye-. No. No! No way!" Kohaku quickly looked up to meet Ruri's gaze.

A gleam of mischief flashed in the older one's eyes. "Hmmm? I don't think there's a problem with that. Why not go on a date with him?"

"Ruri, it's not a date! I'm a cupid. I'm supposed to be doing the opposite thing!" The younger one huffed. "It's also unfair that Luna would be waiting for him for a long time. I don't want to do that to her."

"My dearest sister is so kind!"

"I'm not."

* * *

Kohaku ended up going.

Senku took her to a ramen restaurant. The scientist and his stepfather frequented the place that it wasn't new to her anymore. She liked the ramen there. It was the best, and she wouldn't have discovered it if it weren't for her current target.

Seriously, she was in heaven.

The ramen was so delicious that she almost forgot she was there to complete her mission. The cupid didn't even notice the expressions she was displaying while eating. It was _that_ good.

As much as she was enjoying herself with the food, she still had to do something. She stole a glance to her right and saw that Senku was still eating. Kohaku would admit she was starting to pick up feelings for him. But, she had to push it down and shove it somewhere she wouldn't see or _feel_ it. The window is open now to accomplish her mission. She could risk being discovered as a cupid and erase everyone's memories of her.

Right.

It was now or never.

"Hey, Kohaku," Senku said just as she was about to move. "Can cupids fall in love? Or is it impossible?"

Her eyes widened, but she quickly composed herself.

"I guess it's ten billion percent possible." He chuckled before going back to his ramen.

She looked down at her unfinished bowl. "Did you notice?"

"You're more of an open book than you expected, lioness."

A sigh. "Cupids aren't supposed to fall in love with humans. We're fickle beings."

"It's a no, huh?" He looked at her, and she could see those red eyes fill up with sadness.

"I want to, but I can't let Luna hanging." Her hands clenched under the table. "It would be selfish of me."

"Then, how does it go? How can you make someone fall out of love?" The scientist asked.

"If a cupid shoots you and another person with a love arrow meant for the two of you, the love spell on Luna will gradually fade. It takes time, but that's the only way." The angel explained.

"Hah! then it's easy," Senku smirked at her. "We just need another cupid to shoot both of us."

Kohaku's heart _soared_. She felt something bloom inside her, and the feelings she tried to prevent from spilling overflowed. She could hear the loud thrumming of her heart. It was _too much_.

It didn't help how his words gave her hope. She knew the chances were slim. It would take time for that to happen. Possibly forever even.

But, he makes it sound so easy.

He made it sound _possible_.

Kohaku and Senku smiled at each other. They will try to do something about their situation.

For now, they'll enjoy the day that they could claim for themselves.

* * *

From a few seats away, a bicoloured-haired man chuckled at the scene. He was quite satisfied with the results of his emergency mission. The task was completely unexpected, yet everything turned out well. He had to admit he thought he was going to be discovered by the two. Not that it matters anymore.

The cupid got up from his seat and paid for his ramen. Just as he passed the duo, he smiled. He continued walking and, for the last time, Gen paused to take a final look at them through the glass doors of the restaurant.

"Happy Valentine's Day, you two."

**Notes for the Chapter:**

> Lacie here~!
> 
> I had this AU idea ever since the first day and I've been trying not to use it. The urge was there during the 'first time' prompt/theme. I love Ukyo and I was about to put him here as the cupid who'd get those two together. Gen won inside my mind and that's why we see him in the end. I'm sorry to Luna because of what I did too.
> 
> Anyway, thank you for reading and see you for the last day!


	7. Domestic AU: Unexpected

**Summary for the Chapter:**

> A Domestic AU with a bit of Vampire AU where Senku comes home to see Kohaku having problems.
> 
> For SenHaku Week 2021 Day 7: Hormones.

It would be an understatement to say Senku was happy.

He was _ecstatic_.

His day was going well so far. There was less traffic going to work today. Their science project was successful, and they finished it before the expected date. He even got an email saying his astronaut candidate application was accepted!

The scientist was definitely in a good mood. All he had to do to complete his day was go home to his girlfriend, and have a toast with her. It sounded like he was possessed, but a fine day like this was so rare in his life of 3,715 years.

As he entered the shared apartment, he was expecting to be greeted by Kohaku. There was no sign of her, however. She would usually come running and smiling at him. It was like a daily routine since she always comes home earlier than him. No matter how late it was, she was there without fail.

"Kohaku?" Senku called out.

There was only silence.

The scientist hastily took off his shoes and headed straight to their bedroom. When he opened the door, he caught the faint scent of blood. He frowned once he realized what was going on.

Kohaku was all bundled up on the bed. She wasn't sleeping, but her eyes were closed. She looked so uncomfortable that Senku's previous happiness immediately turned into worry. As he took a seat beside her, the mattress shifted beneath them. She opened her eyes and turned to him.

She weakly smiled at him. "Senku."

"Lioness, I'm home." He helped her sit up then kissed her on the forehead with tenderness. "Can you manage?"

"Ha! I'm not totally-"

Senku watched her curl up in pain as he inserted a finger in his ears. "Uhuh. Sure. It's normal for cramps to be that bad. Have you taken painkillers?"

"I wanted to tough it out."

"I don't think you're ten billion percent successful so far."

"Watch me! I'll be moving like it's nothing!" She groaned. "Just give me five minutes."

"How about I get you cranberry juice and cook while you rest up? " He suggested. Without even waiting for an answer, he patted her back and headed to the kitchen.

He could feel Kohaku keeping an eye on him from the bedroom. Senku knew she wasn't used to him showing emotions. It was so rare for that to happen. He always talked about how emotions were illogical, but he did have his moments.

After living for 3,715 years, he's been through so much as an immortal. There were points in his life he put his emotions in the backseat for logic. It allowed him to excel in the field of science more than he expected.

Kohaku was an unexpected part of his life.

She developed a crush on him, and he shot her down quickly. They remained as friends, but things still changed. He slowly grew to like her. At first, he blamed it on his hormones. He would have stuck to that.

But, he couldn't ignore the fact that vampires react strongly to the blood of their soulmates.

"Weird." He could hear Kohaku whisper before the sheets rustled in the bedroom. Senku chuckled.

Things really do change.

**Notes for the Chapter:**

> A really sleepy Lacie here~.
> 
> I have a lot to say since this is the last chapter. It's my first time participating in a pairing week and completing a series of AUs like this. My only regret was that I was already sleepy by the time I was writing the last theme and I had made it short.
> 
> Anyway, let me tell you about this chapter. This was supposed to be mostly a Vampire AU, but it leaned more towards Domestic AU with how Senku and Kohaku interacted. I was going to put a dinner scene where they were supposed to have a toast, but I'm really sorry. My body yearns so much for sleep that I can't think properly anymore ahahaha. But, yes. This chapter was revolving around the different side of Senku and Kohaku's relationship. I have this headcanon where if Senku was a vampire and lived for a long time, he'd have a rollercoaster of emotions at some point. So, the vampire Senku we have does accept the logic of emotions, but he rarely shows it. It's like he's on guard most of the time.
> 
> Also! I want to take this opportunity to thank everyone who continued to support this series. I'm really grateful to have this kind of reception at least. I wasn't expecting it. So, once again, thank you so much! I really appreciate it!
> 
> To those who wanted more, I want to announce that I might start making an actual series for one of the AUs here. I'm leaning more on the Mission AU, but I still need to get a firmer grasp of the character's personality before I continue. The idea is already in my mind though!
> 
> Thank you for even reading the end notes! I hope everyone has a great day, and remember! You can ship anyone you like! Have a great day~.


End file.
